The invention relates generally to charge coupled devices (CCDs), and more particularly to back illuminated CCDs.
Charge coupled devices (CCDs), which were originally developed as computer memory, are widely used as imaging detectors, in fields ranging from astronomy to medicine. Incident light is converted to electronic charge in a CCD; the charge is collected in potential wells, transferred out, detected, and stored. Images are then produced from the stored data. In a CCD, the electronic circuitry and gates are formed on one side of a silicon wafer, the frontside; the other side of the wafer is the backside.
When illuminated on the frontside, absorption of incident light by the electronic circuitry reduces quantum efficiency. As an alternative, CCDs can be illuminated from the backside; however, backside illumination produces other problems. When the incident photons enter the silicon wafer, they are absorbed and produce electronic charge by the photoelectric effect. The wafer thickness must be sufficient to allow charge generation, and a depletion region should exist to transport the charge to the collecting channels. For conventional low resistivity substrates the thickness of the depletion region is limited to less than about 10 .mu.m. Therefore, for good blue and ultraviolet response, the substrate must be extremely thin, resulting in a very fragile and expensive structure.
Therefore, a backside illuminated CCD is desired which has a thick substrate, and which has a high quantum efficiency over a broad range of wavelengths, from infrared and red to blue and ultraviolet. The CCD should be relatively simple and inexpensive to manufacture. It is desired to have a backside illuminated CCD imaging system for use in astronomy.